This piston ring filer might more appropriately be called a ring grinder, as they most commonly use a thin grinding wheel to touch off the ends of a piston ring to enlarge the ring gap (as installed in the cylinder bore). This very common one is hand cranked and may sell for as little as $32 (if you browse around a bit). More sophistocated power operated models could cost as much as a few hundred dollars.

If you have an engine rebored and are assembling it with new pistons and rings, you may not need to worry about the ring gap, as most new piston rings are precision made and will have the correct ring gap right out of the box. But it is always prudent to check the ring gap anyway. Just place a new piston ring into the cylinder and square it up with a touch of the piston. Then use the thickness gauges to measure the gap at the ends of the ring. For new parts the ring gap is commonly specified as minimum .0025 to .0030 inch per inch of cylinder bore diameter. That makes it .008 inch minimum for most MG engines. For a single engine and a few possible out of spec piston rings, you might just carefully touch off the tips of the rings with a thin grinding wheel in a Dremel tool.

However, .... If you are on a tight budget to refresh an moderately worn engine, and you don't want to have it rebored or buy new pistons, and the pistoins are in decent condition, and the cylinder bores are not worn too badly oversize, then you might need a ring filer to do it right. When the ring gap is too large because of cylinder wear, you can buy the next size larger ring set. With .010" larger diameter rings the circumference will be about .031" longer. The oversize rings will most likely leave you with a gap too small, or even a bit of overlap in the ring tips. Then you can grind the ring gap to suit, and hopefully end up with a gap close to the desired minimum.